1. Field of the invention
The present invention relates to a color thermal transfer recorder in which many heating elements in a thermal head are selectively heated and a color image is formed on a sheet of image receiving paper while an ink sheet and the image receiving paper sheet are supported between the thermal head and a platen and the ink sheet is conveyed at a speed lower than that of the image receiving paper sheet.
2. Description of the Related Art
In a well-known color thermal transfer recorder, an image is generally printed on a sheet of image receiving paper while an ink sheet is conveyed at a speed lower than that of the image receiving paper sheet. In such a recorder, a using amount of the ink sheet can be reduced so that running cost of the recorder can be reduced.
For example, the color thermal transfer recorder of this kind is shown in Japanese Patent Application Laying Open (KOKAI) No. 61-242869. In this patent application, a plurality of ink layers are arranged in the longitudinal direction of an ink sheet and have colors different from each other. The plural ink layers are set to one set of color groups. The ink sheet is used by repeatedly forming the color groups in the longitudinal direction. When an image is printed, this ink sheet is conveyed in its longitudinal direction and ink of each of the ink layers of each of the color groups formed in the ink sheet is transferred onto a sheet of image receiving paper. Thus, a color image is recorded on the image receiving paper sheet.
In this case, the ink sheet has a leading color mark, a heading mark and a terminal end mark. The leading color mark is formed to detect that a leading end region of each of the color groups reaches a printing position between a heating element of a thermal head and a platen. The heading mark is formed to detect that a leading end region of an ink layer of each of colors except for a leading ink layer of each of the color groups reaches the printing position. The terminal end mark is formed to detect a terminal end of the ink sheet. These marks are detected by a detecting means constructed by first and second sensors so that conveyance of the ink sheet is controlled.
The above first and second sensors are generally arranged in predetermined positions in the vicinity of the printing position such that each of the marks can be reliably detected.
As described later, such arranging positions of the first and second sensors are determined in accordance with a pitch between the ink layers in a conveying direction of the ink sheet. It is generally considered that the positions of the first and second sensors must be set to approach the pointing position as this pitch is reduced.
As mentioned above, the ink sheet is conveyed at a speed lower than that of the image receiving paper sheet in the color thermal transfer recorder. In this recorder, a conveying amount of the ink sheet is smaller than that of the image receiving paper sheet. Accordingly, the pitch between the ink layers is reduced in comparison with a recorder in which the ink sheet and the image receiving paper sheet are conveyed at an equal speed. Since the pitch between the ink layers is small, the first and second sensors must be generally arranged very near the printing position for the above reasons.
However, various kinds of constructional elements such as the thermal head, the platen, etc. are arranged in a region near the printing position. Accordingly, it is not easy to arrange the first and second sensors in such a region. Therefore, a degree of freedom in design of the recorder is greatly restricted.